Highly reflective substrates for the digital processing of photopolymer printing plates

ABSTRACT

An improved digitally imageable relief printing element having an increased direct-cure imaging speed upon exposure to lasers and other digital sources of actinic radiation. The printing elements of the invention comprise a reflective layer beneath a photosensitive resin layer so that instead of being absorbed by the reflective layer, photons of actinic radiation are reflected back up into the photosensitive layer, thereby speeding up the curing rate of the printing element.

This application is a continuation of application Ser. No. 10/462,977,filed Jun. 16, 2003, now Pat. No. 7,005,232.

FIELD OF THE INVENTION

This invention relates to an improved digitally imageable reliefprinting elements having an increased direct-cure imaging speed uponexposure to lasers and other digital sources of actinic radiation.

BACKGROUND OF THE INVENTION

Flexographic printing is widely used in the production of newspapers andin the decorative printing of packaging media. In flexographic printing,a layer of a flexible printing medium is deposited onto a flexiblesubstrate such as a thin sheet of steel, aluminum, or synthetic polymer,to form a printing plate or a printing sleeve. A relief patterncorresponding to the negative image to be printed is formed in theprinting medium. The plate is then mounted on the printing press, andprinting commences.

In the manufacture of flexographic printing plates, photosensitiveprinting material is coated onto the substrate to form the printingplate or printing sleeve. The coated side is exposed with light to forma negative of the image to be printed, causing photopolymerization ofthe exposed portion of the printing medium, which then becomesphysically hardened and resistant to solvent removal. The unexposed andtherefore unhardened portion of the printing medium is removed bywashing with solvent, leaving a relief pattern of the image to beprinted. The printing plate is mounted on a press and printingcommences.

Non-flexographic printing plates such as letterpress plates are alsoused for printing newspapers, shoppers, and books. Photosensitive resincompositions have been developed for use with non-flexographic printingapplications for the same reasons disclosed above for flexographicapplications. The use of photosensitive printing media for themanufacture of letterpress printing plates is essentially the same asfor flexographic printing applications.

Direct cure refers to an imaging approach wherein the photopolymer ofthe plate (or other printing element) absorbs some of the actinicradiation causing a chemical reaction that polymerizes (i.e., cures) thephotopolymer, rendering it insoluble in the washout solvent. Coherentenergy, i.e., actinic radiation, is directed onto the surface of thephotosensitive matrix in the desired pattern.

Various means have been proposed to increase the direct-cure imagingspeed of flexographic and letterpress printing elements upon exposure tolasers and other digital sources of actinic radiation. For example,efforts have been made to develop more highly reactive photosensitiveresins. Such materials would be expected to give more completephotoreaction (e.g., crosslinking, dissolution of crosslink bonds,rearrangement, and the like), even with brief laser exposures, as thedesired image is scanned onto the photosensitive resin. One suchreactive photosensitive resin system can be found in U.S. Pat. No.5,976,763 to Roberts et al., the subject matter of which is hereinincorporated by reference in its entirety.

Other efforts have focused on enhancing the imagewise exposure of aphotosensitive material by using an apparatus that subjectsphotosensitive materials to a relatively low energy pre-exposure usingthe electromagnetic energy during the non-imaging portion of theexposure process (i.e., a backscan beam exposure) prior to subjectingthe photosensitive materials to the main imaging exposure (i.e., animagewise exposure). This concept is discussed in U.S. Pat. No.6,262,825 to Mueller et al., the subject matter of which is hereinincorporated by reference in its entirety. However, this processrequires two exposure steps, thus increasing the time needed to processthe photosensitive materials.

The inventors of the instant invention have found that the use of ahighly reflective layer beneath the photosensitive resin layer cangreatly enhance the imaging speed of photopolymer relief printingplates, while maintaining good resolution, when image-wise exposed usingdigital sources of actinic radiation. Instead of being absorbed by thereflective layer, the photons of actinic radiation are reflected back upinto the photopolymer where they speed up the curing of the printingelement.

While reflective layers have not previously been contemplated for use inflexographic or letterpress relief image printing elements, they havebeen suggested for use in other processes.

For example, highly reflective substrates have been proposed for use inproducing image-receiving elements. U.S. Pat. No. 5,380,695 to Chiang etal., the subject matter of which is herein incorporated by reference inits entirety, disclose an image-receiving element comprising a support,wherein the support may comprise transparent, opaque or translucentmaterial, with reflective (opaque) supports being preferred for theproduction of identification documents where image date is viewedagainst an opaque background. There is no suggestion in Chiang et al.that the reflective supports can be used in producing relief imageprinting plates.

Likewise, U.S. Pat. Nos. 5,468,540 and 5,670,096 to Lu, the subjectmatter of which is herein incorporated by reference in its entirety,describe a reflectroreflective article used as a transparent overlay toprotect documents from tampering. Again, there is no suggestion that thereflective layer can be used to produce relief image printing plates.

U.S. Pat. No. 5,636,572 to Williams et al., the subject matter of whichis herein incorporated by reference in its entirety, describes a surfacelayer below the IR-sensitive layer for reflecting IR radiation back intothe IR-sensitive layer in order to increase net energy absorption anddecrease laser power requirements. However, the invention described byWilliams is directed to lithographic printing plates and is concernedwith reflecting IR radiation back into the IR-sensitive layer, insteadof the actinic radiation contemplated by the inventors of the presentinvention.

The inventors of the present invention have found that the benefit ofhigh substrate reflectivity is particular to the imaging of plates withdigital sources where the actinic radiation is substantially coherent,i.e., “high brightness.” Conventional exposure systems, which havenon-coherent sources of actinic radiation, can actually be harmed byhighly reflective substrates due to the substantial scatter of thereflected radiation into non-image areas. The harmful effect of highsubstrate reflectivity is discussed in U.S. Pat. No. 4,622,088 to Min,the subject matter of which is herein incorporated by reference in itsentirety. Min describes that when highly reflective supports are used,oblique rays passing through clear areas in the image-bearingtransparency will strike the surface of the base at an angle other than90° and after reflection, will cause polymerization in the non-imageareas. Min teaches that this disadvantage can be overcome when thephotopolymer layer is on a radiation-reflective support by anintervening stratum sufficiently absorptive of actinic radiation so thatless than 35% of the incident radiation is reflected. The layerabsorptive of reflected radiation or nonradiation scatter layer orantihalation layer, can be made by dispersing a finely-divided dye orpigment which substantially absorbs actinic radiation in a solution oraqueous dispersion of a resin or polymer which is adherent to both thesupport and the photoinsolubilized image and coating it on the supportto form an anchor layer which is dried. This concept is discussed alsoin U.S. Pat. No. 4,460,675 to Gruetzmacher et al. and U.S. Pat. No.4,423,135 to Chen et al., the subject matter of which is hereinincorporated by reference in its entirety.

Furthermore, U.S. Pat. No. 6,037,101 to Telser et al., the subjectmatter of which is herein incorporated by reference in its entiretydiscloses a photosensitive recording material wherein if highlyreflective panels or sheets are used as the substrate, the reflectivepanels or sheets contain suitable antihalation agents, such as carbonblack or manganese oxide. In the alternative, the antihalation agentscan be applied as a separate layer to the substrate or may be present inthe adhesion-promoting layer or in the photopolymer layer.

Thus, there is a clear need in the art for methods that will enhance the“imagewise” exposure sensitivity of photosensitive materials, therebypermitting photoimaging to proceed as rapidly as possible, allowing forthe rapid conversion of the photosensitive materials into finishedarticles. Furthermore, there remains a need to for a digitally imageableflexographic relief printing element that can provide an increaseddirect-cure imaging speed upon exposure to coherent sources of actinicradiation.

SUMMARY OF THE INVENTION

The current invention proposes an improved digitally imageable reliefprinting element having an increased direct-cure imaging speed uponexposure to lasers and other digital sources of actinic radiationcomprising a reflective layer, at least one photocurable layer on top ofsaid reflective layer, and optionally, a removable coversheet on top ofthe at least one photocurable layer, wherein said printing element isimaged using a digital source of actinic radiation.

Also contemplated by the present invention is a method of increasing thedirect-cure imaging speed of a digitally imageable relief printingelement, comprising the steps of providing a photocurable reliefprinting element comprising a reflective layer, at least onephotocurable layer on top of the reflective layer, and optionally, aremovable coversheet on top of said at least one photocurable layer, andexposing said photocurable relief printing element to a source ofactinic radiation to directly cure the photocurable relief printingelement.

The printing elements of the invention comprise a reflective layerbeneath a photosensitive resin layer so that instead of being absorbedby the reflective layer, photons of actinic radiation are reflected backup into the photosensitive layer, thereby speeding up the curing rate ofthe printing element.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The inventors have found that the use of highly reflective substratescan greatly enhance the imaging speed of photosensitive relief printingelements, while maintaining good resolution, when image-wise exposedusing digital sources of actinic radiation. Instead of being absorbed bythe reflective layer, the photons of actinic radiation are reflectedback up into the photosensitive layer where they speed up the curing ofthe printing element.

Direct-cure refers to an imaging approach wherein the photopolymer inthe printing element absorbs some of the actinic radiation, causing achemical reaction that polymerizes, or cures, the photopolymer,rendering it insoluble in the washout solvent that follows. Coherentenergy, i.e., actinic radiation, is directed onto the surface of thephotosensitive matrix in the desired pattern.

Photosensitive materials contemplated for use in the practice of thepresent invention include letterpress printing plates, flexographicprinting plates, and the like. The printing elements are useful in avariety of printing applications including newspapers, inserts,directors, packaging, preprint liners, tags and labels, etc.

The chemistry of the photopolymer resin can be any type, including forexample (meth)acrylate-based resins (see, for example, U.S. Pat, No.5,348,844, which is herein incorporated by reference in its entirety),thiolene-based resins (see, for example, U.S. Pat. No. 3,783,152, whichis herein incorporated by reference in its entirety), vinyl ether-basedresins (see, for example, U.S. Pat. No. 5,446,073, which is hereinincorporated by reference in its entirety), cationic-based resins (see,for example, U.S. Pat. No. 5,437,964, which is herein incorporated byreference in its entirety), diazonium-based resins (see, for example,U.S. Pat. No. 4,263,392, which is herein incorporated by reference inits entirety), and the like, as well as suitable combinations of any twoor more thereof.

The photosensitive materials of the invention can have varyingthicknesses, depending on the end use contemplated for suchphotosensitive materials. For example, for letterpress printingapplications, the thickness of the photosensitive material can vary inthe range from about 5 mils to about 50 mils, with a preferred rangefrom about 10 mils to about 30 mils. For flexographic printingapplications, the thickness of the photosensitive material can vary inthe range from about 8 mils to about 250 mils, with a preferred rangefrom about 100 mils to about 120 mils.

Printing elements of the invention can contain a single layer or amulti-layer of a photoimageable photosensitive resin on top of a highlyreflective layer, and may optionally include a protective coversheet.The printing elements can take the shape of a sheet or a cylinder, andcan be formed by extrusion, lamination, or casting. These techniques canreadily be carried out by those skilled in the art.

Printing elements of the invention generally have a relief thickness ofabout 0.5 microns up to about 800 microns, preferably from 300 to about500 microns, and are processable using water, solvent, or thermalblotting techniques, depending on the particular chemistry of thephotosensitive resin system.

In order to produce the relief printing plates of the instant invention,the photosensitive resin material is subjected to a coherent beam ofelectromagnetic energy, in the spectral range to which thephotosensitive material is reactive, to conditions sufficient togenerate, from the coherent beam, an imaging beam capable of causingreaction of the photosensitive material.

The benefit of using highly reflective layers (substrates) is particularto the imaging of plates with digital sources where the actinicradiation is substantially coherent, i.e., “high brightness.”Conventional exposure systems, which have non-coherent sources ofactinic radiation, can actually be harmed by highly reflectivesubstrates due to the substantial scatter of the reflected radiationinto non-image areas. As previously discussed, in conventional exposuresystems, an intervening stratum or other means must be used to preventharmful effects of a highly reflective layer.

The inventors have found beneficial results in the practice of theinvention by using substrates with greater than 40 percent reflectivityat the wavelength(s) emitted by the actinic source where thephotoinitiator has an absorption. Preferably, substrates with greaterthan 60 percent reflectivity at the wavelength emitted by the actinicsource where the photoinitiator are used. The reflectance of thesubstrate can be diffuse or specular (i.e., mirror-like). Preferably,the reflectance of the substrate is more specular.

High reflectivity can be imparted to the reflective layer in a varietyof ways. For example, the support can be inherently reflective, whereinthe support is made from aluminum, zinc and/or tin. Another means ofimparting high reflectivity is by applying a reflective metal coating tothe surface of the support by means of vacuum deposition or the like,wherein a layer of aluminum, tin, zinc and/or silver is deposited onto ametal or plastic support. If one of these methods is used to impart highreflectivity, a clear primer layer may optionally be used on top of thereflective substrate or reflective layer for purposes of adhering thesupport to the photopolymerizable layer or for other reasons known tothose skilled in the art.

Another means of imparting high reflectivity is through the use ofhighly reflective pigments such as aluminum, mica, and/or bismuth, whichcan be formulated into a primer coating and coated onto a metal orplastic support. The highly reflective primers contemplated for use inthe invention may be aqueous-based, solvent-based, TV-curable, or powdercoated primers. The primers can be applied by means of Meyer bars, rollcoating, curtain coating, extrusion, spraying, or slot dies. Othermethods would also be known to one skilled in the art.

The source of actinic radiation can be either a laser or a non-laser.Non-limiting examples of lasers usable in the invention include sourcescapable of providing coherent electromagnetic energy of suitable energyto promote imaging of photosensitive materials via reflection orrefraction of the electromagnetic energy, e.g., ion gas lasers (e.g.,argon ion lasers, krypton lasers, helium:cadmium lasers, and the like),solid state lasers (e.g., Nd:YAG, frequency-doubled Nd:YAG lasers, andthe like), semiconductor diode lasers, molecular gas lasers (e.g.,carbon dioxide lasers, and the like), and the like, and suitablecombinations of two or more thereof. Such laser sources are generallycapable of emitting electromagnetic energy in the spectral range towhich the photosensitive material is reactive. Further, theelectromagnetic energy emitted by the laser source is capable ofoperating as an imaging beam to directly write image data onto thephotosensitive material. Non-limiting examples of non-laser sourcesinclude plasma lamps, xenon lamps, mercury lamps, and carbon arc lamps.

The preferred wavelength of the source of actinic radiation is from 250to 500 nanometers, preferably from 320 to 420 nanometers. Preferredwavelengths are those which correspond to the spectral sensitivity ofthe photoinitiator being employed.

The invention will now be described by reference to the followingnon-limiting examples:

INVENTION EXAMPLE 1

Primer mixing: In the order given, 50.00 parts of NeoRez R-966polyurethane aqueous dispersion (Zeneca Resins Inc), 25.00 parts ofQW18-1 polyurethane resin (K. J. Quinn & Co), 0.50 parts of SilwetL-7600 polydimethylsiloxane (Osi Specialties Inc), 0.25 parts ofSilquest A-187 silane (Osi Specialties Inc), 7.75 parts of deionizedwater, and 0.50 parts of Nopco DSX-1550 (Henkel Corp) were mixed at roomtemperature for 15 minutes. 16.00 parts of BiFlair 83S bismuth pigment(EM Industries) was added and mixed for an additional 15 minutes.

-   Substrate coating: A length of 0.0066 inch thick tin-free steel was    pretreated by sequentially washing with 0.1 N aqueous sodium    hydroxide and deionized water, then dried with hot air. The primer    composition was applied via roll-coating to the cleaned steel to a    wet thickness of 25–40 microns. The sheet was dried at 400 F for 75    seconds. The average percent reflectance of the coated substrate was    measured to be 66% over the wavelength range of 340–390 nm on a    Shimadzu UV-2102 PC UV/Vis spectrophotometer equipped with ISR-260    integrating sphere.-   Resin mixing: Part A: 7.63 parts of Kraton D1107 block copolymer    (Kraton Chemical Co) was dissolved in 6.36 parts of lauryl acrylate    (Sartomer Co) by stirring at 45° C. for one hour. Part B: 10.17    parts of polyoxyalkylene mono-phenyl ether (Dai-Ichi Kogyo Seiyaku    Co. Ltd.), 5.92 parts of Dabco XDM (Air Products Inc), 6.36 parts of    polyethylene glycol diacrylate (SR-344 by Sartomer Co) and 7.63    parts of ethoxylated trimethylolpropane triacrylate (SR-499 by    Sartomer Co) were blended together. Added to Part B next were 0.25    parts of butylated hydroxy toluene (Sherex Chemical Inc), 1.32 parts    of 1-hydroxycyclohexyl phenyl ketone (Irgacure 184 by Ciba), 0.26    parts of diphenyl (2,4,6-trimethylbenzyl)phosphine oxide (Lucerin    TPO by BASF Corporation), and 0.20 parts of zinc diacrylate (SR-705    by Sartomer Co).

53.90 parts of particulate emulsion copolymer composed ofbutadiene/methacrylic acid/divinylbenzene/methacrylate (TA906 by JSRCorporation, see EP 0 607 962 A1, U.S. Pat. No. 6,140,017), 13.99 partsof Part A and 32.11 parts of Part B were mixed in a Moriyama Mixer(Model D3-7.5 Moriyama Mfg Works, Ltd.) at 80 C. Part B was introducedto the mixer in seven separate and equal aliquots. The resin was mixeduntil homogeneous.

-   Plate making: The photosensitive resin described above was passed    through a single screw extruder and sheet die at approximately 80 C    to apply a 15 mil thick layer onto a length of pre-coated substrate.-   Plate exposing and processing: The entire plate was pre-sensitized    with an overall bump exposure. The dose of bump exposure used is    determined empirically because it varies depending on the resin    formula. The preferred bump exposure is 90% of the maximum exposure    possible before the appearance of photopolymer residue on the plate    processed under normal processing conditions. After the bump    exposure, the plate was image-wise exposed with an Innova 300 argon    ion UV laser (Coherent Inc) at an exposure level necessary to hold a    3% highlight dot on a 100 lines per inch halftone screen. The    imaging beam had a 1/e² spot diameter at the plate surface of    approximately 25 microns and a full angle beam divergence of    approximately 10 milli-radians.

The imaged plate was then processed in a NAPPflex FP-II processor (NAPPSystems Inc.) using deionized water at 140 F., a spray pressure of 850psi, and a conveyor speed of 28 inches per minute. The plate made inInvention Example 1 required an imaging exposure of 42 mj/cm² in orderto hold the targeted 3% highlight dot. See Table 1.

INVENTION EXAMPLE 2

-   Primer mixing procedure: In the order given, 50.00 parts of NeoRez    R-966 polyurethane aqueous dispersion (Zeneca Resins), 15.50 parts    of MP4983R/40R copolymer emulsion (Michelman Inc), 0.30 parts of    Nopco DSX-1550 (Henkel Corp), 0.70 parts of Silwet L-7600    polydimethylsiloxane (Osi Specialties Inc) and 25.50 parts of    deionized water were mixed for 15 minutes at room temperature. 8.0    parts of Aquavex 1752-207S aluminium pigment (Silberline) was added    and mixed a further 15 minutes.-   Substrate coating: The same procedure was used as shown in    Example 1. The average percent reflectance of the coated substrate    was measured to be 73% over the wavelength range of 340–390 nm on a    Shimadzu UV-2102 PC UV/Vis spectrophotometer equipped with ISR-260    integrating sphere.

Plate making: The same procedure was used as shown in Example 1. Theplate made in Invention Example 2 required an imaging exposure of 35mj/cm2 in order to hold the targeted 3% highlight dot. See Table 1.

INVENTION EXAMPLE 3

-   Primer mixing procedure: In the order given, 45.00 parts of NeoRez    R-966 polyurethane dispersion (Zeneca Resins), 2.50 parts of Alcogum    SL-76 acrylic emulsion (National Starch & Chemical), 1.6 parts of    sodium hydroxide, 50.4 parts of deionized water and 0.50 parts of    Surfynol 440 surfactant (Air Products & Chemical Inc) were mixed at    room temperature for 15 minutes.-   Substrate coating: The same procedure was used as shown in Example 1    except that a sheet of 10 mil thick aluminum was used as the    substrate. The average percent reflectance of the coated substrate    was measured to be 75% over the wavelength range of 340–390 nm on a    Shimadzu UV-2102 PC UV/Vis spectrophotometer equipped with ISR-260    integrating sphere.-   Plate making: The same procedure was used as shown in Example 1. The    plate made in Invention Example 3 required an imaging exposure of 35    mj/cm² in order to hold the targeted 3% highlight dot. See Table 1.

COMPARATIVE EXAMPLE 4

-   Primer mixing: The same primer and procedure was used as shown in    Example 3. Substrate coating: The same metal and procedure was used    as shown in Invention Example 1. The average percent reflectance of    the coated substrate was measured to be 34% over the wavelength    range of 340-390 nm on a Shimadzu UV-2102 PC UV/Vis    spectrophotometer equipped with ISR-260 integrating sphere.-   Plate making: The same procedure was used as shown in Invention    Example 1. The plate made in Comparative Example 4 required an    imaging exposure of 70 mj/cm² in order to hold the targeted 3%    highlight dot. This plate could not be imaged nearly as rapidly as    those in the invention examples. See Table 1.

TABLE 1 Average % Exposure Dose to Reflectance hold 3% dot at 100 @340–390 nm lpi screen (mj/cm²) INVENTION EXAMPLE 1 66 42 INVENTIONEXAMPLE 2 73 35 INVENTION EXAMPLE 3 75 35 COMPARATIVE EXAMPLE 4 34 70

1. A method of direct curing a relief printing element with a source ofactinic radiation, comprising the steps of: a. providing a photocurablerelief printing element comprising: i. a reflective layer whichreflective layer has a reflectivity of at least 40 percent at thewavelength(s) emitted by the source of actinic radiation; ii. at leastone photocurable layer on top of said reflective layer, and iii.optionally, a removable coversheet on top of said at least onephotocurable layer; and b. exposing said photocurable relief printingelement to the source of actinic radiation to directly cure thephotocurable relief printing element; where the source of actinicradiation comprises a laser such that at least 40 percent of the actinicradiation hitting the reflective layer is reflected back into thephotocurable layer, thereby curing the photocurable layer.
 2. A methodaccording to claim 1, wherein the photocurable layer comprises aphotopolymer and a photoinitiator.
 3. A method according to claim 1,wherein the reflective layer has a reflectivity of at least 60 percentat the wavelength emitted by the actinic source and at least 60 percentof the actinic radiation hitting the reflective layer is reflected backinto the photocurable layer thereby curing the photocurable layer.
 4. Amethod according to claim 1, wherein the source of actinic radiation isa laser selected from the group consisting of ion gas lasers, solidstate lasers, semiconductor diode lasers, molecular gas lasers, andcombinations of the foregoing.
 5. A method according to claim 1, whereinthe source of actinic radiation has a wavelength between about 250nanometers and about 500 nanometers.
 6. A method according to claim 1,wherein the source of actinic radiation has a wavelength between about320 nanometers and about 420 nanometers.
 7. A method according to claim1, wherein the reflective layer is selected from the group consisting ofa reflective support and reflective coating on a support.
 8. A methodaccording to claim 7, wherein the reflective layer is a reflectivesupport selected from the group consisting of aluminum, zinc, tin andmixtures of the foregoing.
 9. A method according to claim 7, wherein thereflective layer is a reflective coating on a support and the reflectivecoating is selected from the group consisting of aluminum, tin, zinc,silver, and mixtures of the foregoing, deposited onto the support bymeans of vacuum deposition.
 10. A method according to claim 9, whereinthe support is selected from the group consisting of metal and plasticsupports.
 11. A method according to claim 1, wherein said printingelement is selected from the group consisting of flexographic printingelemonts and offset printing elements.